dh_python3: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/dh_python3 - from package: dh-python

You may install the required package with command apt-get install {package}

Error in other languages:
dh_python3: bevel nie gevind nie
dh_python3: comandă negăsită
dh_python3: comando não encontrado
dh_python3: commande introuvable
dh_python3: command not found
dh_python3: käsku ei ole
dh_python3: không tìm thấy lệnh
dh_python3: komanda nerasta
dh_python3: Kommando nicht gefunden.
dh_python3: kommandot finns inte
dh_python3: komut yok
dh_python3: nie znaleziono polecenia
dh_python3: níor aimsíodh an t-ordú
dh_python3: no se encontró la orden
dh_python3: no s'ha trobat l'ordre
dh_python3: opdracht niet gevonden
dh_python3: parancs nem található
dh_python3: perintah tidak ditemukan
dh_python3: príkaz nenájdený
dh_python3: příkaz nenalezen
dh_python3: команда не найдена
dh_python3: командата не е открита
dh_python3: 命令找不到
dh_python3: コマンドが見つかりません